I received a request from John Nielsen-Gammon to answer several questions that he passed on from retired NASA Personnal and Associates. I have reproduced the relevant parts below, as these may be of general interest. The website of the NASA group can be found here [http://www.therightclimatestuff.com/]

On Mon, Nov 5, 2012 at 5:27 PM, John Nielsen-Gammon wrote:

Roger –

Hi!

I’m working with a group of retired NASA folks and associates who are taking an independent look at climate change. They recently came across your climate blog manifesto, and I offered to pass a couple of questions along to you. To wit:

1. When you say “humans are significantly altering the global climate”, what do you mean by significant? More than nature? Just enough to be detectable? etc.

My Reply

Significant, as I intend it, means a clear detectable signal. For example, the increase of the atmospheric concentration CO2 at Mauna Loa resulting from fossil fuel activity is significant. Similarly, land use change has resulted in a clear signal of a change in the partitioning of the surface fluxes of heat and and moisture. Human input of aerosols have a well documented signal in the atmosphere and though surface deposition.
These are examples of a significant alteration of the global (and regional and local) climate by humans. There, are of course, many other examples.

2. Do you consider cloud and moisture feedbacks to fall into the human category or the natural category?

My Reply

To the extent that feedbacks are different due to human climate forcings, this part of the feedback is, of course, human caused. However, I do not know how to separate feedbacks from observations, although, of course, you can in models.

3. What are the other important human-associated climate forcings besides CO2?

Pielke Sr., Roger A., 2008: A Broader View of the Role of Humans in the Climate System is Required In the Assessment of Costs and Benefits of Effective Climate Policy. Written Testimony for the Subcommittee on Energy and Air Quality of the Committee on Energy and Commerce Hearing “Climate Change: Costs of Inaction” – Honorable Rick Boucher, Chairman. June 26, 2008, Washington, DC., 52 pp. https://pielkeclimatesci.files.wordpress.com/2010/01/testimony-written.pdf

See also the assessments

National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Committee on Radiative Forcing Effects on Climate Change, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., 208 pp. http://www.nap.edu/openbook/0309095069/html/

“In addition to greenhouse gas emissions, other first-order human climate forcings are important to understanding the future behavior of Earth’s climate. These forcings are spatially heterogeneous and include the effect of aerosols on clouds and associated precipitation [e.g., Rosenfeld et al., 2008], the infl uence of aerosol deposition (e.g., black carbon (soot) [Flanner et al. 2007] and reactive nitrogen [Galloway et al., 2004]), and the role of changes in land use/land cover [e.g., Takata et al., 2009]. Among their effects is their role in altering atmospheric and ocean circulation features away from what they would be in the natural climate system [NRC, 2005]. As with CO2, the lengths of time that they affect the climate are estimated to be on multidecadal time scales and longer.”